Thyristor and thermal switch device and assembly techniques therefor

ABSTRACT

A device may include a lead frame, where the lead frame includes a central portion, and a side pad, the side pad being laterally disposed with respect to the central portion. The device may further include a thyristor device, the thyristor device comprising a semiconductor die and further comprising a gate, wherein the thyristor device is disposed on a first side of the lead frame on the central portion. The device may also include a positive temperature coefficient (PTC) device electrically coupled to the gate of the thyristor device, wherein the PTC device is disposed on the side pad on the first side of the lead frame; and a thermal coupler having a first end connected to the thyristor device and a second end attached to the PTC device.

BACKGROUND Field

Embodiments relate to the field of surge protection devices, and moreparticularly to overvoltage protection devices and resettable fuses.

Discussion of Related Art

Thyristors are widely used in alternating current (AC) power controlapplications. As with other semiconductor switches, a thyristorgenerates heat when conducting current. Often a device or apparatus thatincludes a thyristor includes other components to measure or manage heatgeneration caused by the thyristor, including heat sinks, cooling fans,or temperature sensors to monitor the thyristor body temperature.Lacking temperature control, a thyristor may enter a thermal runawaystate, leading to catastrophic failure. In cost-sensitive applications,large heat sinks or cooling fans are not an acceptable solution, forcingdesigners to specify operation of a thyristor well below its maximumoperational rating to prevent heat-related failure.

With respect to these and other considerations, the present disclosureis provided.

SUMMARY

Exemplary embodiments are directed to improved protection devices. Inone embodiment, a device may include a lead frame, where the lead frameincludes a central portion, and a side pad, the side pad being laterallydisposed with respect to the central portion. The device may furtherinclude a thyristor device, the thyristor device comprising asemiconductor die and further comprising a gate, wherein the thyristordevice is disposed on a first side of the lead frame on the centralportion. The device may also include a positive temperature coefficient(PTC) device electrically coupled to the gate of the thyristor device,wherein the PTC device is disposed on the side pad on the first side ofthe lead frame; and may include a thermal coupler having a first endconnected to the thyristor device and a second end attached to the PTCdevice.

In another embodiment, a method may include providing a lead frame, thelead frame having a central portion and a side pad, where the side padis laterally disposed with respect to the central portion. The methodmay further include affixing a thyristor device to a first side of thecentral portion of the lead frame, the thyristor device comprising asemiconductor die and further comprising a gate. The method may alsoinclude affixing a positive temperature coefficient (PTC) device to theside pad on the first side of the lead frame, and connecting a first endof a thermal coupler to the thyristor device and a second end of thethermal coupler to the PTC device, wherein the gate of the thyristordevice is electrically connected to the PTC device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A presents an exploded perspective view of a device in accordancewith the present embodiments;

FIG. 1B presents a top plan view of a device in assembled form accordingto embodiments of the disclosure;

FIG. 1C provides a circuit depiction of one implementation of the deviceof FIG. 1A;

FIG. 2A presents a top plan view of an arrangement of devices at a firststage of assembly;

FIG. 2B presents a top plan view of an arrangement of devices at asecond stage of assembly;

FIG. 2C presents a top plan view of an arrangement of devices at a thirdstage of assembly;

FIG. 3 depicts an exemplary graph showing characteristics of anexemplary PTC device; and

FIG. 4 shows a flow for assembly of an apparatus according to someembodiments of the disclosure.

DESCRIPTION OF EMBODIMENTS

The present embodiments will now be described more fully hereinafterwith reference to the accompanying drawings, in which exemplaryembodiments are shown. The embodiments are not to be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey their scope to those skilled in the art. In thedrawings, like numbers refer to like elements throughout.

In the following description and/or claims, the terms “on,” “overlying,”“disposed on” and “over” may be used in the following description andclaims. “On,” “overlying,” “disposed on” and “over” may be used toindicate that two or more elements are in direct physical contact withone another. Also, the term “on,”, “overlying,” “disposed on,” and“over”, may mean that two or more elements are not in direct contactwith one another. For example, “over” may mean that one element is aboveanother element while not contacting one another and may have anotherelement or elements in between the two elements. Furthermore, the term“and/or” may mean “and”, it may mean “or”, it may mean “exclusive-or”,it may mean “one”, it may mean “some, but not all”, it may mean“neither”, and/or it may mean “both”, although the scope of claimedsubject matter is not limited in this respect.

In various embodiments, a hybrid device is provided including athyristor device such as a TRIAC (triode for alternating current), aswell as a PTC device acting as a thermal switch. The apparatus may bearranged in a configuration that provides improved design, facilitatingeasier assembly as well as better integration into devices to beprotected.

As used herein the term “thyristor device” may include a singlethyristor, a silicon controlled rectifier (SCR) or a TRIAC device. As isknown, a thyristor device is related to a silicon controlled rectifier,where an SCR is composed of a layered structure having an arrangement ofN-type semiconductor regions or layers as well as P-type semiconductorlayers or regions, in a four layer sequence of P-N-P-N, for example. Ina thyristor, a gate is connected to an inner layer of the four-layerdevice. A TRIAC may be considered a type of thyristor where currentconduction may take place in both directions, as opposed to a singlethyristor that conducts current in just one direction. A TRIAC may betriggered by applying either a positive or a negative voltage to thegate. Once triggered, TRIACs continue to conduct, even if the gatecurrent ceases, until the main current drops below a certain levelcalled the holding current.

The present embodiments provide improvements over known devices byintegrating a positive temperature coefficient (PTC)-type component intoa compact device to control operation of a thyristor device. FIG. 1Apresents an exploded perspective view of a device 100 in accordance withthe present embodiments, while FIG. 1B presents a top plan view of thedevice 100 in assembled form according to embodiments of the disclosure.Additionally, FIG. 1C provides a circuit depiction of one implementationof the device 100 of FIG. 1A. As shown in FIG. 1A, the device 100includes a lead frame 102, where the lead frame 102 includes a centralportion 104, and a side pad 106, where the side pad 106 is laterallydisposed with respect to the central portion 104. The lead frame 102 maygenerally be composed of an electrically conductive material such as ametal, including a copper material, such as pure copper or copperalloys. The lead frame 102 may also include a first main terminal lead112, a second main terminal lead 114, and a gate lead 116, as shown. Thesecond main terminal lead 114 may be connected to the central portion104. The device 100 may further include a thyristor device 124. Thethyristor device 124 may be formed in a semiconductor die as a multiplelayer structure where different layers have different doping polarity asin to known thyristor devices. The thyristor device 124 may be embodiedas a TRIAC, where the thyristor device 124 includes a gate. As suggestedin FIG. 1A, when assembled, the thyristor device 124 is disposed on afirst side 150 of the lead frame (upper side of lead frame as shown inFIG. 1A) 102 on the central portion 104. As such, the second mainterminal lead 114 may be electrically connected to the thyristor device124 via a second main terminal electrode (not shown) of the thyristordevice 124 on a lower surface opposite to an upper surface 129.

As shown in FIG. 1A, the device 100 includes a positive temperaturecoefficient (PTC) device 122, which device may be a polymer PTC devicein various embodiments. Such a device may be capable of changingresistivity by a factor of 10,000 or so over a narrow range oftemperatures, such as over 10 degrees C. or 20 degrees C. As discussedfurther below, when assembled, the PTC device 122 may be electricallycoupled to the gate of the thyristor device 124, and in particular maybe disposed on the side pad 106 on the first side 150 of the lead frame102. As further depicted in FIG. 1A, the device 100 may include athermal coupler 128, that when assembled, has a first end 130 connectedto the thyristor device 124 and a second end 132 attached to the PTCdevice 122. The thermal coupler 128 may be a highly thermally conductivebody, such as a copper arm, that provides a high thermal conduction pathbetween the thyristor device 124 and the PTC device 122. As such, whenthe thyristor device 124 heats up during operation, the PTC device 122may rapidly heat up in consequence, obtaining a temperature close to thetemperature of the thyristor device 124.

As further shown in FIG. 1A the device 100 may include a main terminalcontact clip 126, arranged to contact a first main terminal electrode127 while not contacting the copper arm, that is, while not touching thethermal coupler 128. The main terminal contact clip 126 when assembledmay provide an electrically conductive path between first main terminallead 112 and the first main terminal (see MT1 of FIG. 1C) of thethyristor device 124. As such, during operation of the device 100,electrical current may be conducted between first main terminal lead112, through the thyristor device 124 and to second main terminal lead114. As detailed below, during operation, current flow between these twoleads may be regulated by current supplied through gate lead 116.

When assembled, the thermal coupler 128 may also serve as an electricalconnection between a gate electrode 134 disposed on the upper surface129 of the thyristor device 124 and the PTC device 122. As such, duringoperation, the PTC device 122 is in electrical series between the gatelead 116 and the gate electrode 134. In this manner the PTC device 122may serve as a current regulator for gate current supplied to thethyristor device 124. For example, under moderate current conditions ofoperation where current conducted by the thyristor device 124 is notexcessive, the temperature of the thyristor device 124 may remain belowa maximum junction temperature T_(jmax), as defined in the art. Inparticular, if the junction temperature T is increased above T_(jmax),the leakage current may be high enough to trigger the thyristor'ssensitive gate. The thyristor device will then have lost the ability toremain in the blocking state and current conduction will commencewithout the application of an external gate current.

The value of T_(jmax) may be determined according to a specific TRIAC orthyristor device, as known in the art. For example, T_(jmax) may be 110°C., 120° C., 130° C., or 140° C., or 150° C. in some examples. Theembodiments are not limited in this context. The PTC device 122 may bedesigned to operate as a relatively low electrical resistance devicewhen temperature is below T_(jmax). Accordingly, adequate gate currentmay flow through an electrical circuit from gate lead 116 to gateelectrode 134, providing for the targeted operation of the thyristordevice 124. When temperature approaches T_(jmax) or reaches T_(jmax),the PTC device 122 may be designed with a trip temperature close to theT_(jmax), wherein the PTC device 122 operates as a relatively highelectrical resistance device. In one specific embodiment, a PTC device122 may be designed with a trip temperature of 130° C., while a T_(jmax)for a TRIAC used as thyristor device 124 is 150° C. In this manner,current may be starved from the gate of the thyristor device 124,causing the current conducted by the thyristor device 124 to be reduced,limiting further heating of the thyristor device 124. This limiting ofcurrent may be such that damage is prevented to the thyristor device 124and surrounding components, where such damage may otherwise occur in adevice lacking the PTC device 122 under runaway thermal conditions. Asfurther shown in FIG. 1A the device 100 may include a heat sink 138,providing a thermal connection between the lead frame 102 and externalheat sinks. The device 100 may also include ceramic substrate 136,connected by solder 140 to heat sink 138 and providing electricalisolation between lead frame 102 and the heat sink 138. The heat sink138 is exposed to outside conditions and is connected to an outside heatsink (not shown), which heat sink may be an outer case of a finalproduct.

Turning to FIG. 1B there is shown an assembled view of a variant ofdevice 100, where the device 100 also includes a housing 144, not shownin FIG. 1A. The housing 144 may be a solid molded material as in knowndevices, that encases the other components described hereinabove. Inoperation, the device 100 may be conveniently coupled to any circuit orset of devices or components to be regulated, such as in ACapplications. FIG. 1C provides a circuit representation of oneimplementation for device 100, where the device 100 includes a TRIAC asthe thyristor device 124. As such, current regulation may take place intwo opposite directions through the TRIAC with the aid of the PTC device122.

FIG. 2A presents a top plan view of an arrangement 200 of devices 202 ata stage of assembly that may be termed a first stage of assembly (notthe beginning of assembly). In this example, the devices 202 may besubstantially the same as device 100 described hereinabove. Thearrangement 200 include a framework that houses multiple devices duringassembly, where singulation of the devices (see FIG. 1B for example of asingulated device) may take place after assembly is complete. At thestage of assembly in FIG. 2A, a thyristor device 124 is affixed to thecentral portion 104 of a lead frame 102, for example, by soldering. Thegate electrode 134 and first main terminal electrode 127 are disposed onthe upper surface 129, facing outwardly (upwardly) and not in directelectrical contact with the central portion 104 that holds the thyristordevice 124. Additionally, a second main terminal electrode (not shown)on a lower surface of the thyristor device 124 is in electrical contactwith the central portion 104. In addition, a PTC device 122 is connectedto the side pad 106, for example, by soldering.

FIG. 2B presents a top plan view of the arrangement 200 of devices at asecond stage of assembly. At this stage of assembly, a thermal coupler128 is in place, acting to electrically connect the PTC device 122 tothe thyristor device 124, as well as to thermally couple the PTC device122 to the thyristor device 124. The thermal coupler 128 may be solderedto the PTC device 122, and may be in the form of a metal clip thatadheres to the gate electrode 134, and may be attached to the gateelectrode by soldering or wire bonding. In addition, a first mainterminal contact clip 126 is in place and may be soldered to the firstmain terminal lead 112. The first main terminal contact clip 126 isarranged to contact the first main terminal electrode 127, for example,using a solder connection, while not contacting the thermal coupler 128.In the embodiment shown, the first main terminal contact clip 126 has aC-shape, so as to provide extensive surface area to couple to the firstmain terminal electrode 127, while not contacting the gate electrode134. Other shapes for the first main terminal electrode 127 arepossible. Additionally, in the embodiment of FIG. 2B, the thermalcoupler 128 may extend above the plane of the first main terminalcontact clip 126, so as to extend over the first main terminal contactclip 126 while not touching the first main terminal contact clip 126, asshown. Other shapes for thermal coupler 128 are possible, for example,the right angle shape of the embodiment of FIG. 1A.

FIG. 2C presents a top plan view of the arrangement 200 of devices 202at a third stage of assembly, where a given set of components to from afinal device is encapsulated in a housing 204, such as an epoxymaterial. For example, a thyristor device together with correspondingPTC device are encapsulated in a given housing, while the gate lead 116,first main terminal lead 112, and second main terminal lead 114 extendoutside of the housing 204. Subsequently, the arrangement 200 may besingulated to form a set of individual devices as shown in FIG. 1B.

In some embodiments, the PTC device 122 may be sized so as to fit on theside pad 106 in a manner that does not extend beyond the side pad 106.In some embodiments the PTC device 122 may have the shape of a rectangleor square in the plan view of FIG. 2A, where the size of the PTC deviceis 60 mil×60 mil or less. The embodiments are not limited in thiscontext.

In variants of the above embodiments, the electrical characteristics ofthe PTC device 122 may be tailored to the electrical characteristics ofa thyristor device 124 to be protected. As an example, a given PTCdevice may be characterized by a trip temperature. The trip temperaturecorresponds to a temperature that separates a low temperature state(conduction state) where electrical resistance is relatively lower andincreases relatively slowly with increased temperature, from a hightemperature state (high impedance state) where electrical resistancebecomes relatively much higher and increases relatively rapidly withincreased temperature. FIG. 3 provides an exemplary graph showingresistance of an exemplary 200 mil square PTC test chip as a function oftemperatures, where resistance is plotted on a logarithmic scale. Inthis example, a trip temperature may be designated at approximately 125°C. Below this temperature, the resistance is below a few Ohms andincreases by just one order of magnitude over a 100 C range between 25 Cand 125° C. Above the trip temperature the resistance increases by fourorders of magnitude (10⁴) over just 10° C. Accordingly, when current isbeing conducted through the PTC chip under initially “cold” conditionswhere temperature is less than 125° C., and the temperature of the PTCchip subsequently increases and exceeds 125° C. by just several degrees,the current passing through the PTC chip may be reduced by many ordersof magnitude, other things being equal. These characteristics may beusefully harnessed by coupling the PTC chip of FIG. 3 to a thyristordevice as described above, such as a TRIAC, where the TRIAC has amaximum junction temperature in the range of, for example, 125° C. to145° C. In this manner, the temperature of the TRIAC, may be indirectlycontrolled by reduction of gate current supplied via the gate terminaland the PTC chip, when the TRIAC temperature exceeds 125° C. Thislimiting prevents a current/thermal runaway scenario, by limiting theduration and value of the maximum temperature experienced by the TRIACto a targeted maximum level, such as T_(jmax). As such, control of theoperation of the TRIAC device is maintained. Additionally, the presenceof the PTC chip allows the TRIAC device to operate close to or atT_(jmax), an improvement over prior known TRIAC device operation whereoperating temperature may be limited to much lower temperatures toprevent thermal runaway and potential damage to devices.

Merely as an illustrative example, in the conduction state of a TRIAC, agate driver may apply up to 12V when a gate current of 100 mA is needed.This condition leads to an impedance maximum of 120 Ohm. In a blockingstate, a maximum of 20 V may be applied and the gate current is to belimited to less than 0.1 mA to maintain a TRIAC in an OFF state at atemperature of 125° C. In this circumstance the impedance called for is200 kOhm or higher. Such change in resistance over more than threeorders of magnitude may be readily achieved by a 60 mil×60 mil PTCdevice arranged according to the present embodiments. Notably, theimpedance values shown for FIG. 3 using a 200 mil×200 mil PTC chip scaleup by a factor of approximately 11 for a 60 mil×60 mil die.

Notably, to improve the design of a device such as the device 100, thethermal coupling between a PTC device 122 and a thyristor device 124 maybe taken into account. For example, a T_(jmax) or other design limit ofa thyristor device may be 130° C. Under operation of the devicestructure of device 100, the thermal coupling between PTC device 122 andthyristor device 124 may such that when the thyristor device reaches 130C, the temperature of the PTC device 122 as disposed on the side pad 106may reach 110° C. Accordingly, the PTC device 122 may be arranged tohave a trip temperature in the range of 110° C. so that a high impedancestate is triggered when the PTC device 122 exceeds 110° C.,corresponding to the condition when the thyristor device 124 temperatureexceeds 130° C.

FIG. 4 illustrates a process flow 400 according to additionalembodiments of the disclosure. At block 402, a lead frame is provided,where the lead frame has a central portion and a side pad, where theside pad is laterally disposed with respect to the central portion. Invarious embodiments the lead frame may include three leads where onelead is connected to the central portion.

At block 404, a thyristor device, such as a TRIAC, is affixed to a firstside of the central portion of the lead frame. The thyristor device mayinclude a semiconductor die that has a gate. A gate may include a gatecontact formed on an upper surface of the semiconductor die.

At block 406, a PTC device is affixed to the side pad on the first sideof the lead frame. The PTC device may be soldered to the side pad, forexample. At block 408, a thermal coupler is connected to the thyristordevice on a first end and is connected to a PTC device on a second end.The thermal coupler may serve to electrically connect the thyristordevice to the PTC device while also providing a high thermal conductancebetween the PTC device and thyristor device.

While the present embodiments have been disclosed with reference tocertain embodiments, numerous modifications, alterations and changes tothe described embodiments are possible while not departing from thesphere and scope of the present disclosure, as defined in the appendedclaims. Accordingly, the present embodiments are not to be limited tothe described embodiments, and may have the full scope defined by thelanguage of the following claims, and equivalents thereof.

1. A hybrid device, comprising: a lead frame, the lead frame comprising:a central portion; and a side pad, the side pad being laterally disposedwith respect to the central portion; a thyristor device, the thyristordevice comprising a semiconductor die and further comprising a gate,wherein the thyristor device is disposed on a first side of the leadframe on the central portion; a positive temperature coefficient (PTC)device electrically coupled to the gate of the thyristor device, whereinthe PTC device is disposed on the side pad on the first side of the leadframe; and a thermal coupler having a first end connected to thethyristor device and a second end attached to the PTC device.
 2. Thehybrid device of claim 1 wherein the lead frame comprises a coppermaterial.
 3. The hybrid device of claim 1, wherein the thermal couplercomprises a copper arm, the copper arm electrically connecting the gateto the PTC device.
 4. The hybrid device of claim 3, wherein thethyristor device comprises an upper surface, the upper surface includinga gate electrode and a first main terminal electrode, wherein the firstmain terminal electrode surrounds the gate electrode and is electricallyisolated from the gate electrode.
 5. The hybrid device of claim 4,further comprising a main terminal contact clip, arranged to contact thefirst main terminal electrode while not contacting the copper arm. 6.The hybrid device of claim 4, the lead frame further comprising: a firstmain terminal lead, electrically coupled to the first main terminalelectrode of the thyristor device; a second main terminal lead,electrically coupled to a second main terminal electrode of thethyristor device on a lower surface opposite the upper surface; and agate lead, electrically coupled to the PTC device.
 7. The hybrid deviceof claim 6, wherein the copper arm connects to a first surface of thePTC device, and wherein the gate lead connects to a second surface ofthe PTC device, the second surface being opposite the first surface. 8.The hybrid device of claim 1, wherein the thyristor device comprises aTRIAC.
 9. The hybrid device of claim 1, wherein the PTC device comprisesa polymer PTC.
 10. The hybrid device of claim 1, wherein the PTC devicecomprises a trip temperature, wherein the thyristor device comprises amaximum junction temperature, and wherein the maximum junctiontemperature exceeds the trip temperature by 20 degrees C. or less.
 11. Amethod for forming a hybrid device, comprising: providing a lead frame,the lead frame comprising: a central portion; and a side pad, the sidepad being laterally disposed with respect to the central portion;affixing a thyristor device to a first side of the central portion ofthe lead frame, the thyristor device comprising a semiconductor die andfurther comprising a gate; affixing a positive temperature coefficient(PTC) device to the side pad on the first side of the lead frame; andconnecting a first end of a thermal coupler to the thyristor device anda second end of the thermal coupler to the PTC device, wherein the gateof the thyristor device is electrically connected to the PTC device. 12.The method of claim 11, wherein the thermal coupler comprises a copperarm, the copper arm electrically connecting the gate to the PTC device.13. The method of claim 12, further comprising: providing a gateelectrode and a first main terminal electrode on an upper surface of thethyristor device, wherein the first main terminal electrode surroundsthe gate electrode and is electrically isolated from the gate electrode;and affixing a main terminal contact clip to the first main terminalelectrode while not contacting the copper arm or the gate electrode. 14.The method of claim 13, further comprising: connecting a first mainterminal lead to the main terminal contact clip; coupling a second mainterminal lead to a second main terminal contact of the thyristor deviceon a lower surface opposite the upper surface; and connecting a gatelead to the PTC device.
 15. The method of claim 14, wherein theconnecting the second end of the thermal coupler to the PTC devicecomprises connecting the thermal coupler to a first surface of the PTCdevice, and wherein the connecting the gate lead to the PTC devicecomprises connecting the gate lead connects to a second surface of thePTC device, the second surface being opposite the first surface.
 16. Themethod of claim 11, wherein the thyristor device comprises a TRIAC. 17.The method of claim 11, wherein the PTC device comprises a polymer PTC.